1. Technical Field
The embodiments described herein relate to a semiconductor memory device, and more particularly, to a data output circuit of a semiconductor memory device.
2. Related Art
In general, in a semiconductor memory device, specifically, a double data rate (DDR) SDRAM, data input and output operations are performed in synchronism with the rising edges, as well as the falling edges, of a clock so that double data input and output speeds can be obtained. In addition, conventional data output circuits of a semiconductor memory device output data through a main driver to a pad and include a pre-driver, which receives input with rising data and falling data, to drive the main driver.
A slew rate is an index that indicates how quickly the voltage level of an output signal changes with respect to time, and can be defined as having a slope that indicates the amount of change in a voltage level per unit time. For example, as the slew rate increases, the slope of the output signal increases and the level change of the signal quickly occurs. However, if the slew rate is too large, the generation of noise increases. Conversely, if the slew rate is too small, the generation of noise decreases, but the skew of data increases.
Accordingly, the pre-driver is configured to adequately adjust the slew rate of the data output by the main driver. In the conventional art, RC delay control using resistors and capacitors is commonly adopted in the pre-driver to change the turn-ON timing of the main driver, so that the slew rate of the data can be adjusted. However, because the RC delay control adjusts the turn-ON timing of the main driver with a certain amount of time delay, it is difficult to implement precise control in conformity with changes in environmental variables, such as process, voltage and temperature (PVT). Furthermore, the RC delay control is difficult to be applied for high speed input and output operations, and since the bandwidth of the pre-driver decreases due to the RC delay, jitter is likely to occur in the output of the main driver.